but ive heard it used for other things. Cant it EVER be used for non-em waves? Would it e incorect?

It depends on whether or not you are using c to mean the speed of light in a vacuum. If you are then you can't use it as that isn't the speed of the wave. If you are using it as a general speed of a wave and not ##3 \times10^8## then you could use it in the same way as you could use any other symbol, however using c does risk confusion.